The invention relates generally to a disable circuit that stops the ignitor function of a high intensity discharge (HID) lamp ignition circuit. More particularly, the invention relates to an apparatus and method to control the timing and triggering of the disable function of the igniter circuit.
High intensity discharge (HID) lamps such as metal halide (MH) and high pressure sodium (HPS) lamps have increasingly gained acceptance over incandescent and fluorescent lamps for commercial and industrial applications. HID lamps are more efficient and more cost effective than incandescent and fluorescent lamps for illuminating large open spaces such as construction sites, stadiums, parking lots, warehouses, and so on, as well as for illumination along roadways. An HID lamp comprises at least an arc-tube containing two electrodes, chemical compounds and a fill gas. The fill gas can comprise one or more gases. To initiate operation of the lamp, the fill gas is ionized to facilitate the conduction of electricity between the electrodes.
HID lamps can be difficult to start. An HID lamp such as a conventional HPS lamp uses a 2500 to 4000 volt pulse at least once per half-cycle and at selected times during the cycle in order to start, as set forth in a number of standards such as ANSI C78.1350 on HPS lamps, for example. An ignitor is used to provide the necessary pulses to start the conventional HID lamp. If the lamp is extinguished after lamp operation has elevated lamp temperature, the lamp cannot be restarted until after the lamp cools down and the fill gas can be ionized again. For many types of HID lamps, this lamp cooling period can be between approximately 40 seconds and 2.5 minutes, which can be considered unacceptable in situations where, for example, emergency lighting is desired.
A number of circuits have been developed to start or hot restrike HID lamps. These ignitors generally include resistors, pulse transformers and other components, in addition to a conventional ballast. These devices can reduce system efficiencies and substantially increase system cost.
An exemplary ignitor 100 is depicted in FIG. 1. Terminals 102 and 104 of a lighting unit are connected to an AC power source 106, as well as to a ballast 108 and a lamp 110. The ballast 108 comprises a tap 112 and two winding portions 114 and 116. The ignitor 100 has terminals which are connected to terminals 102, 112 and 110. A charging circuit for hot restarting a high pressure xenon HPS lamp or other HID lamp having similar hot restart requirements is provided which comprises a semiconductor switch 118 such as a silicon-controlled rectifier (SCR) or the like is connected so that one end of its switchable conductive path is connected to the end of the first portion 116 of the ballast. The other end of the conductive path of the SCR 118 is connected to the tap 112 via a storage capacitor 120. A number of sidacs 122 or other breakdown devices are connected between the gate and the anode of the SCR 118. A current-limiting resistor 126 is provided in series with the sidacs 122 and 124. If the voltage on the capacitor 120 increases to a level which reaches or exceeds the threshold voltage of the breakdown devices 122 and 124, the sidacs 122 and 124 become conductive, placing the SCR 118 in a conductive state. Accordingly, the capacitor 120 discharges through the portion 18 of the ballast. Because the winding portions 114 and 116 of the ballast are electromagnetically coupled, the portion 116 of the ballast operates as the primary of a transformer in that a voltage is induced in the winding portion 114. The high voltage generated in the winding portion 114 of the ballast 108 is imposed on the lamp 110. The relationship of the winding portions 114 and 116 is selected to create a voltage using the SCR 118 and the sidacs 122 and 124 which is sufficiently high to ionize the material within the arc tube of the lamp 110.
With further reference to FIG. 1, a charging circuit 144 for the capacitor 120 is connected between the tap 112 and the terminal 102 at the other side of the AC power source 106. This charging circuit preferably comprises two diodes 128 and 130, a pumping capacitor 132 and two radio frequency chokes 134 and 136 connected in series between the tap 112 and the terminal 102. Two diodes 138 and 140 are connected between the capacitors 120 and 132 and are poled in the opposite direction from the diodes 128 and 130.
The charging circuit 144 depicted in FIG. 1 provides for the controlled, step-charging of the storage capacitor 120. During one half cycle of the AC power source 106, a current flows through the chokes 134 and 136, the capacitor 132 and the diodes 128 and 130 to charge the capacitor 132. The capacitor 132 is selected to be relatively smaller than the capacitor 120 (e.g., 0.047 microfarads (xcexcF) versus 5 xcexcF). On the next half cycle of the AC power source 106, the capacitor 120 is charged and the voltage across the capacitor 132 increases the incoming half wave from the AC power source 106 so as to provide energy on the order of 2.7 microjoules to the storage capacitor 120. Since the capacitor 120 requires more energy due to its relative size, the capacitor 120 can be provided with energy from both the incoming AC signal and the capacitor 132 in one cycle. On the next half cycle, the capacitor is charged again and delivers energy to the capacitor 120 again on the subsequent half cycle. Thus, the charge on the capacitor 120 is increased with each alternate half cycle using a pumping action.
When the capacitor 120 reaches the breakdown voltage of the sidacs 122 and 124, the sidacs become conductive and therefore render the SCR 118 conductive. The capacitor 120 therefore discharges through the portion 116 of the ballast 108 to generate a high voltage in the portion 114 of the ballast. The large magnitude of the capacitor 120 discharges significantly more energy into the magnetic field of the ballast 108 as compared with a conventional HID lamp ignitor and therefore excites the ballast 108 to a relatively high degree. The highly excited ballast 108, with its corresponding collapsing magnetic field, pushes the lamp into a discharge state and therefore a low impedance state so that the discharge state can be maintained by the normal AC power source 106. The discharging capacitor 120 produces current flow which is in the same direction as the continued current flow produced by the collapsing field, and which is provided through the lamp as the SCR 118 is turned off by the instantaneous back voltage bias placed on the capacitor 120 by the same collapsing field energy. The resistor 152 can be connected in series with the SCR 118 to cause the peak of the high voltage pulse to be lower and the base (i.e., width) of the pulse to be longer. The resistor 152 limits the high voltage and therefore reduces dielectric stress to allow the use of lower cost magnetic components.
The ignitor 100 depicted in FIG. 1 further comprises an HPS lamp starting circuit comprising a capacitor 146 connected in series with a resistor 148 and a sidac 150 or similar breakdown device. The resistor 148 is connected to the junction between the inductors 134 and 136 and the capacitor 132. The ignitor 100 comprises a current-limiting resistor 152 in series with the parallel combination of the SCR 118 and the sidacs 122 and 124.
The above-mentioned HID lamps should be provided with a disabling circuit such that, if the lamp fails to start, the disabling circuit would discontinue the hot or cold strike used to initiate the HID lamp. This feature is useful in prolonging the life expectancy of the ignitor, helps protect the ballast system, and provides the ability to apply HID ignitors to harsh and hazardous environments.
Accordingly, a need exists for a reliable means of disabling the ignitor portion of a HID lamp, and an accurate method to time when the disablement of the ignitor occurs. Further, a need exists for a power supply for proper operation of semiconductor devices used in the disabling circuitry, and a solid state contact in the lamp circuit that will not release sparks when actuated by the disabling circuit.